Why am I getting such "good" framerates?

Yes, there is an fps point on each system that will not be smooth to the eye, no matter what you do. To me the trick is to find that point and slowly increase until you can find the lowest fps that are consistently smooth. Maybe kick it up a notch but leave it alone.

The point I'm trying to make is above a certain point (20fps?) - if it's smooth - increasing fps will not make it smoother and in some cases can actually induce stutters.

It's a balancing act between software settings, add ons and hardware. Once you get it - leave it alone!  

Vic